Identification of Colon-Typhoid Bacteria. 25 



(This work is a part of the investigation of food poisoning, 

 conducted under the direction of Dr. M. J. Rosenau, professor of 

 preventive medicine and hygiene, Harvard Medical School. The 

 investigations are done under the auspices of the Advisory Com- 

 mittee of the National Research Council on the Toxicity of 

 Preserved Foods, and under a grant to Harvard University from 

 the National Canners' Association.) 



13 (1473) 



On methods of isolation and identification of the members of 

 the colon-typhoid group of bacteria. Further studies on 

 C. R. indicator. 



By J. Bronfenbrenner, D. Soletsky and M. J. Schlesinger. 



[From the Department of Preventive Medicine and Hygiene, Harvard 



Medical School.] 



Some time ago we described an indicator for the direct measure- 

 ment of the hydrogen ion concentration in growing bacterial 

 cultures. 1 This indicator consists of the mixture of China blue 

 and rosolic acid, and covers the range of changes in the hydrogen 

 ion concentration between C H = 1 X io~ 9 and C H = 5 X io~ 5 . 

 The choice of the dyes was made on the basis of their possessing 

 suitable turning points coupled with the fact that their respective 

 phases of highest color lie on the opposite sides of the point of 

 neutrality. China blue is colorless at the concentration of hydro- 

 gen ions below I X io~ 7 , and gives graded intensity of blue with 

 the increase in the hydrogen ion concentration up to the point 

 of about C H = 5 X io~ 5 , when it reaches its maximum color. 

 Rosolic acid, on the other hand, gives graded intensities of pink 

 beginning with the hydrogen ion concentration close to 1 X io~ 7 , 

 and increasing gradually with the increase of the hydroxyl ions 

 concentration reaching its maximum color at the C OH = 1 X io~ 5 

 (or C H = 1 X io~ 9 ). With the increase in concentration of hydro- 

 gen ions above 1 X io -7 the rosolic acid has a pale yellow (straw) 

 color, which is masked by the color of ordinary culture media. 



1 J. Bronfenbrenner, Jour. Med. Res., XXXIX, I, 25, 191 8. 



